Optical add-drop multiplexer (OADM) technology substantially reduces the cost of Dense Wavelength Division Multiplexing (DWDM) optical networks. An example of a conventional OADM configuration within a DWDM system is shown in FIG. 1. In the conventional system 100 shown in FIG. 1, a multi-channel optical signal 102 is delivered to the input port 103 of OADM device 120. The optical signal comprises a plurality of channels, each comprising a different unique wavelength range, where each channel is denoted by its respective center wavelength, λ1, λ2, λ3, etc. A first optical filter 106 a is used to remove or “drop” one of the incoming multiple channels, e.g., λ1, and to pass through the remaining “express” channels λ2, λ3, and λ4  as signals 104. A second optical filter 106b is used to add a channel λ1′ into the optical path containing the express channels. The express channels exit from OADM 120 apparatus together with the added channel λ1 ′ as a single combined signal 112 at the output port 105.
An example of an optical communications network system containing a conventional OADM is demonstrated in FIG. 2. In the conventional network system 200, a plurality of channels λ1, λ2, . . . ,λN are transmitted between end locations 202a and 202b. The optical network comprises end locations 202a and 202b, a plurality of n intermediate locations, or “nodes” 206.1–206.n disposed between the end locations, and a sequence of optical fiber spans 208.1–208.(n+1) optically connecting the nodes 206.1–206.n and the end locations 202l –202b to one another in a single chain. The first end location 202a comprises a WDM multiplexer (MUX) 204a that combines the channels from separate input paths into a single combined signal that is delivered to a first span 208.1 of optical fiber. Likewise, the second end location 202b comprises a WDM de-multiplexer (DEMUX) 204b that receives a set of combined channels from the last span 208.(n+1) of optical fiber and separates these channels into separate output paths.
Optical signals λi, λi, λi′,and λj′ are added and/or dropped from the chain of optical fiber spans 208.1–208.(n+1) at each of the nodes 206.1–206.n. Each node is disposed between two such consecutive spans of optical fiber and the optical fiber spans join the nodes to one another. Each of the nodes 206.1–206.n comprises a respective one of a set of n OADMs 210.1–210.n that performs the adding and dropping of channels at the node. The OADMs are required in order to allow each of the nodes access to a respective portion of the network traffic while, at the same time maintain the integrity of other channels. Without such OADMs, all channels would have to be terminated at each intermediate node even for a small portion of traffic exchange.
One characteristic of the conventional OADM structure shown in FIG. 1 is that the added channel λ′1, generally comprises an optical power that is different from the powers of the express channels. This power difference arises because the added channel originates from a different optical path from those of the express channels and thus generally incurs a unique set of insertion loss along this path. This unequal-power characteristic does not impose any negative impact to the early local (e.g., “metropolitan” or “metro”) multi-channel OADM systems wherein no optical amplifiers are used. However, the trend of late is to widely deploy amplifiers in such metro OADM systems in order to stretch the link distance and reach more customers. If channels in such an optical network have differing power levels, the weak signals could quickly dissipate after passing through a chain of amplifiers, due to the gain competition of the amplifiers. Therefore, the use of conventional OADM apparatus within a metro optical network also comprising optical amplifiers presents some problems.